1. Field of the Invention
This invention relates to anion exchange resins for separating boron isotopes. More specifically, it relates to anion exchange resins for separating boron isotopes, which have an aminopolyol group as a functional group and have been treated with a specific alkali solution.
2. Description of the Prior Art
Boron is found in nature in such proportion that about 20% is Boron 10 (.sup.10 B) and about 80% is Boron 11 (.sup.11 B), of which Boron 10 has excellent characteristics as an absorber for neutron generated in nuclear reactions and hence used as a neutron abosrbing material such as control rods in various nuclear reactors and thus is an essential substance in the nuclear industry.
However, as described above, since .sup.10 B is present in nature in a proportion of about 20% and the rest is .sup.11 B having almost no neutron absorbing ability, it is necessary to separate and concentrate .sup.10 B from natural boron which is a mixture of .sup.10 B and .sup.11 B isotopes in order to efficiently absorb neutron and control it in a nuclear reactor etc.
One known method for separating boron isotopes comprises effecting separation by ion exchange chromatography using a plurality of ion exchange columns packed with ion exchange resins, in particular, since styrenic chelate type ion exchange resins having an aminopolyol of the following general formula [I]: ##STR1## wherein n is an integer of 1-6 and R represents a hydrogen atom, an alkyl group of 1-5 carbon atoms or a group: --CH.sub.2 [CH(OH)].sub.m --CH.sub.2 OH wherein m is 0 or an integer of 1-6 which has high selectivity on boron as a functional group have a higher isotope separation coefficient (.alpha..sub.11.sup.10) represented by the following equation [II]: ##EQU1## as compared with the values with other conventional strongly basic anion exchange resins and weakly basic anion exchange resins, the above method is an interesting method. As resins useful in the present invention, Diaion CRB02 (tradename, produced by Mitsubishi Chemical Industries, Ltd.) and Amberlite IRA-743 (produced by Rohm & Haas Co.) have hitherto been on market. Of those, Amberlite IRA-743 (old name: XE-243) is used in the boron isotope separation described in French Pat. No. 1520521.
However, the resins of this type generally have slow reaction rates for adsorption and desorption of boric acid, and further in the separation of boron isotopes, the reaction rate for isotope exchange between .sup.10 B and .sup.11 B is slow. In other words, the value of HETP (Height Equivalent of a Theoretical Plate) which is a measure representing an isotope exchange reaction rate expressed by the following equation [III]: ##EQU2## wherein R.sub.1 and R.sub.2 each represents a proportion of isotopes at positions L.sub.1 and L.sub.2 of the boron isotope concentration band was high and therefore the method for separating boron isotopes using said chelate type anionic ion exchange resins could not be accepted as a particularly excellent separating method.
The present inventors have been studying on a method for separating and concentrating boron isotopes using an anion exchange resin having an aminopolyol group as a functional group and have come to discover that by raising the operational temperature, the ion exchange rate of boric acid is enhanced, the HETP is reduced and the solubility of boric acid is increased. However, it has also been discovered that when said ion exchange resin is maintained at high temperatures for a prolonged time, the functional group undergoes thermal deterioration to cause a fluctuation in the amount of the boric acid adsorbed and a fluctuation in the resin volume and therefore stable chromatograph development is hampered.